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Evidence from ophiolites, blueschists, and ultrahigh-pressure
metamorphic terranes that the modern episode of subduction
tectonics began in Neoproterozoic time
Robert J. Stern
University of Texas, Dallas, Geosciences Department, P.O. Box 830688, 2601 North Floyd Road, Richardson,
Texas 75083-0688, USA
ABSTRACT
Earth is the only known planet with subduction zones and plate tectonics, and this fact
demonstrates that special conditions are required for this mode of planetary heat loss.
Sinking of cold, dense lithosphere in subduction zones is the principal plate-driving force,
so plate tectonics could not have begun until Earth cooled sufficiently to allow lithosphere
to collapse into the underlying asthenosphere. Direct geologic evidence for when the modern episode of subduction tectonics began focuses on the first appearance of ophiolitic
graveyards, blueschist facies metamorphic rocks, and ultrahigh-pressure metamorphic terranes. Ophiolites manifest two modes of lithospheric motion expected from subduction
tectonics: seafloor spreading and obduction. High-pressure, low-temperature metamorphic
blueschists and ultrahigh-pressure terranes indicate subduction and exhumation of oceanic
and continental crust, respectively. These lines of evidence indicate that the modern style
of subduction tectonics began in Neoproterozoic time. This revolution in the functioning
of the solid Earth may have driven wild fluctuations in Earth’s climate, described under
the ‘‘snowball Earth’’ hypothesis. These conclusions may be controversial, but suggest
fruitful avenues for research in geodynamics and paleoclimate.
Keywords: plate tectonics, Neoproterozoic, ophiolites, blueschist, ultrahigh-pressure metamorphism, subduction.
INTRODUCTION
Earth is the only known planet with subduction zones and plate tectonics (Stevenson,
2003), and it is controversial why it is so peculiar. It is also controversial when Earth’s
modern tectonic regime began: some argue
that it began early (Parman et al., 2001;
Smithies et al., 2003), and others argue that
subduction began in the second half of Earth
history (Davies, 1992; Hamilton, 2003). In the
following I summarize evidence from subduction proxies—ophiolites, blueschists, and
ultrahigh-pressure metamorphic terranes—and
conclude that the modern style of subduction
tectonics began in Neoproterozoic time. Such
a profound tectonic change should also have
affected Earth’s atmosphere and hydrosphere,
and the radical changes documented for Neoproterozoic climate may have been triggered
by the Neoproterozoic tectonic revolution.
GEODYNAMIC CONSIDERATIONS
Earth’s mantle convection is now driven by
the sinking of cold dense lithosphere at subduction zones. Mantle plumes are a subordinate convection mode (Davies and Richards,
1992). Negative buoyancy of the lithosphere
results from the small increase in density that
silicates undergo as they cool, coupled with
the thickening of the thermally defined lithosphere with age. Lithosphere becomes denser
than the underlying asthenosphere within
;20–50 m.y. (Davies, 1992). Of the force
needed to drive the plates, ;90% comes from
the sinking of subducting lithosphere; the other 10% comes from ridge push (LithgowBertelloni and Richards, 1995). The base of
the continents may be accelerated or retarded
by circulating asthenosphere (Silver and Holt,
2002), but overall Cenozoic plate motions are
well predicted from the distribution of lithosphere age—and thus the mass excess—in
subduction zones (Conrad and LithgowBertelloni, 2002, 2004). Mantle tomography
shows that subducted lithosphere may sink
through the 660 km discontinuity and into the
deep mantle (Grand et al., 1997; Kárason and
van der Hilst, 2000), demonstrating that aging
lithosphere develops a density excess that
takes as long to dissipate as to develop. In
recognition of this modern understanding of
plate-driving forces, Earth’s modern tectonic
style is best termed ‘‘subduction tectonics’’
(Stern, 2004).
The interior of the early Earth was much
hotter than it is today. Many geoscientists assume that this hotter Earth resulted in a more
vigorous plate tectonic regime. Given the geo-
dynamic considerations presented here, this
assumption should be challenged. It would be
more difficult in a hotter Earth to establish the
density inversion required for subduction, because the mantle lithosphere should thicken
more slowly and the oceanic crust would be
much thicker (Pollack, 1997; Sleep, 2000).
Sleep (2000) recognized three modes of heat
loss in silicate planets: magma ocean, plate
tectonics, and stagnant lid. Subduction tectonics can be shut down either by ridge lock, because the Earth’s mantle is too cool to melt
by adiabatic decompression, or by trench lock,
when a hotter mantle generates oceanic crust
that is too thick to subduct. Sometime in the
past, subduction tectonics was prohibited by
trench lock, and sometime in the future it will
stop because of ridge lock. Davies (1992) concluded that the Earth did not cool sufficiently
to allow the lithosphere to subduct until ca.
1 Ga.
In summary, it is contrary to our modern
understanding of plate-driving forces to argue
that a hotter Precambrian Earth favored subduction tectonics. It is more logical to conclude that Earth had to cool sufficiently to allow subduction to begin. The test of this
hypothesis should come from the geologic
record. This record is examined in following
sections.
q 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; July 2005; v. 33; no. 7; p. 557–560; doi: 10.1130/G21365.1.
557
WHAT RECORDS OF SUBDUCTION
SHOULD BE CONSIDERED?
Which kinds of geologic evidence are most
compelling? Direct evidence of subduction
should be paramount. Specifically, three distinctive petrotectonic associations require subduction: ophiolites, blueschists, and ultrahighpressure (UHP) metamorphic terranes.
Ophiolites manifest two modes of lithospheric
motion expected from subduction tectonics:
infinite extension (seafloor spreading) and
convergence (required for obduction). A
growing proportion of the geologic community acknowledges that many ophiolites form
during the earliest stages of subduction and
are preserved as forearc basement (Gurnis et
al., 2004; Robertson, 2004; Shervais et al.,
2004; Stern, 2004). Some ophiolites formed in
backarc basins, and these testify indirectly to
subduction. The rarest type of ophiolite forms
at a mid-ocean ridge, but because seafloor
spreading at mid-ocean ridges is driven by
subduction, these also testify indirectly that
subduction occurred. Where ophiolites form is
controversial; fortunately the tectonic setting
of ophiolite formation is not important for this
analysis because all three tectonic settings for
ophiolites require subduction for formation
and emplacement.
High-pressure (P), low-temperature (T)
metamorphic rocks, especially blueschists and
UHP terranes, also manifest subduction.
Blueschists require unusually cold upper mantle geotherms, only found today in subduction
zones (van Keken et al., 2002). UHP terranes,
especially those forming coesite or diamond
(A-type UHP terranes of Maruyama et al.,
1996), require that siliceous sediments or continental crust was carried down 100 km or
more and then exhumed. UHP terranes most
likely form when continental crust is deeply
subducted during collision and subsequently
exhumed (Liou et al., 2004).
Paleomagnetic data could constrain when
the continents first moved independently.
However, Scotese (2004) noted that the uncertainty in paleomagnetically determined positions increases rapidly with age and concluded that the pre–late Neoproterozoic
paleomagnetic record does not reliably constrain the relative position or motions of the
continents. For this reason, paleomagnetic
data are not used in the following analysis,
although such data may ultimately prove to be
a fruitful way to test my conclusions.
If the presence of ophiolites, blueschists,
and UHP terranes are diagnostic of subduction, then evaluating when subduction tectonics began can be determined by when these
rocks first become common in the geologic
record. Erosion can destroy part of this record,
especially ophiolites, but blueschists and UHP
terranes are metamorphic rocks that are not
558
likely to be eroded away. It will be shown that
the three lines of evidence give a consistent
answer, and this result implies that the record
is not fatally biased by erosion.
OPHIOLITE GRAVEYARDS
The age distribution of ophiolites has been
interpreted to suggest that seafloor spreading
began in Late Archean time (Kusky et al.,
2001), but Archean ophiolites are controversial and rare. Some ophiolites were generated
and emplaced ca. 2.0–1.95 Ga (Kontinen,
1987; Scott et al., 1992), but ophiolites are
uncommon in the rest of the Paleoproterozoic
and Mesoproterozoic record. Paleoproterozoic
ophiolites may represent a short-lived or
aborted episode of subduction tectonics, but
this is exceptional in the pre-Neoproterozoic
record. In general, a few ophiolitic rocks of a
given age do not demonstrate establishment of
a global and long-lived system of subduction
tectonics. It is more important to determine
when ophiolites became common, as revealed
by abundant and widely distributed ophiolites
or ophiolite graveyards.
It was not until almost Neoproterozoic time,
ca. 1000 Ma, that unequivocal ophiolites were
produced, emplaced, and abundantly preserved. The oldest are the ca. 1030 Ma ophiolites of central Asia (Khain et al., 2002, 2003),
which slightly predate the beginning of Neoproterozoic time. Neoproterozoic ophiolites
are especially common in northeast Africa and
Arabia (Dilek and Ahmed, 2003). Neoproterozoic ophiolites are also abundant in central
Asia, graveyard of the paleo–Asian Ocean
(Badarch et al., 2002; Khain et al., 2002,
2003). Smaller regions of Neoproterozoic
ophiolites are found in the Americas, West Africa, Europe, and China. Since first becoming
common in Neoproterozoic time, ophiolites
have become hallmarks of our planet (Dilek,
2003). Furthermore, Moores (2002) concluded
that ophiolites older than ca. 1 Ga were fundamentally different from younger ophiolites,
in particular by having much thicker crust, and
thicker oceanic crust impedes subduction
(Sleep, 2000). The ophiolite record is thus
consistent with geodynamic arguments that
Earth wasn’t cold enough for subduction to be
securely established as the dominant tectonic
mode until relatively recently.
BLUESCHIST CONUNDRUM
Blueschist is a metamorphosed mafic rock
containing abundant sodic amphibole, which
is stable under high-P and low-T conditions
(Maruyama et al., 1996). Blueschists are synonymous with B-type UHP terranes of Maruyama et al. (1996) and are characteristic of
Pacific-type orogenic belts (Ernst, 2003). The
perception that blueschist only forms in subduction zones is based on the association of
blueschists with ancient subduction mélanges
and is confirmed by studies of active subduction zones (Abers, 2005; Maekawa et al.,
1995; Zhang et al., 2004). The best-studied
blueschists, those of the Franciscan and Sanbagawa terranes, appear to have been subducted to depths of 15–70 km before rising
back to the surface (Ernst, 2003).
It has been recognized for almost half a
century that blueschists are not found in very
ancient rocks (de Roever, 1956), and it is now
widely acknowledged that the oldest blueschists date from Neoproterozoic time, ca.
800–700 Ma. These are widely distributed in
West Africa, India, and western China (Maruyama et al., 1996). Somewhat older (ca. 940
Ma) blueschist may exist in Jiangnan, south
China (Shu and Charvet, 1996). One might
think that the absence of blueschists from the
pre-Neoproterozoic record would long ago
have convinced the geoscientific community
that the modern episode of subduction tectonics began in Neoproterozoic time, but this is
not the case. Instead, the geoscientific community generally agrees that the absence of
blueschist from the pre-Neoproterozoic record
results from a hotter pre-Neoproterozoic
Earth.
The consensus for why blueschists are
missing from the first 3.7 b.y. of Earth history
must be reevaluated. The ‘‘hotter Earth’’ explanation implies that the ambient mantle thermal structure controls subduction-zone thermal structure, an assumption that is not
supported. Subduction-zone thermal structure
is controlled by five variables: (1) convergence rate, (2) thermal structure of the subducted plate (a function of slab age), (3) geometry of the subducted slab, (4) rate of shear
heating (5 shear stress 3 convergence rate),
and (5) vigor and geometry of flow in the
mantle wedge (Peacock, 2003). The first two
parameters—convergence rate and slab age—
are paramount. These define the subductionzone thermal parameter (Kirby et al., 1991),
which controls subduction-zone thermal structure, as evinced by the maximum depth of
earthquakes in a given convergent margin
(Molnar et al., 1979). The temperature of ambient mantle and the global geothermal gradient are not thought to be important.
It may be that the hotter early Earth resulted
in plates that were thinner than modern plates
of a given age, and this would result in a hotter subduction zone where blueschist might
not be stable. This possibility can be tested in
modern subduction zones, which can be subdivided into those subducting old (older than
65 Ma) cold lithosphere and those subducting
young (younger than 40 Ma) warm lithosphere
(Peacock and Wang, 1999). Warm subduction
may be an appropriate analog for subduction
on the early Earth (if this occurred). Lowvelocity zones consistent with blueschist faGEOLOGY, July 2005
cies subducted crust are inferred seismically
(Abers, 2005; Zhang et al., 2004) and petrologically (Maekawa et al., 1995) to exist in
cold subduction zones. We need to determine
whether blueschists can form in warm subduction zones.
If all pre-Neoproterozoic subduction zones
subducted warm young lithosphere, blueschist
may not have formed. This explanation is unattractive because subduction of only warm
young lithosphere is unlikely. Subduction occurs because gravitationally unstable lithosphere sinks back into the mantle, and young
warm lithosphere—associated with thickened
oceanic crust—should be buoyant. Somewhere, either downdip or along strike, subduction of young lithosphere must be associated with old, gravitationally unstable
lithosphere that can sink. If Earth’s lithosphere
were sufficiently cold to sink, it should have
formed blueschist somewhere.
UHP TERRANES
The term UHP terrane used here is synonymous with A-type UHP terranes (Maruyama
et al., 1996), which are characteristic of
Alpine-type orogenic belts (Ernst, 2003).
Metamorphic assemblages in UHP terranes include coesite and/or diamond and indicate
peak conditions of ;700–900 8C and 3–4
GPa (Ernst and Peacock, 1996). Such low-T,
high-P conditions are only found today in subduction zones. UHP terranes represent subduction of continental crust to depths of 100–
125 km as a result of continent-continent collision and subsequent exhumation (Liou et al.,
2004). Only a few such UHP terranes have
been documented, at least eight that are coesite bearing and five that are diamond bearing
(Maruyama and Liou, 1998). The oldest, reliably dated UHP locality is the Gourma area
of northwest Mali, which contains coesite and
was metamorphosed ca. 620 Ma (Jahn et al.,
2001). The oldest diamond-bearing UHP terrane is in Kazakhstan, the slightly younger
Kokchetav massif, which contains diamond
and coesite in a dismembered paragneiss and
was metamorphosed at pressures of .4 GPa
(.120 km depth) ca. 530 Ma (Maruyama and
Liou, 1998). The first evidence for deep subduction of continental crust thus is found in
late Neoproterozoic rocks. This implies that
the search for pre-Neoproterozoic UHP terranes should be intensified.
DISCUSSION
The temporal distribution of ophiolites,
blueschists, and UHP terranes is consistent
with the hypothesis that the modern episode
of subduction tectonics began in Neoproterozoic time. This hypothesis cannot be proved,
but further work in key areas should move the
discussion forward. There are at least two
intriguing corollaries of this hypothesis.
GEOLOGY, July 2005
The first concerns when each of the three
subduction-related petrotectonic assemblages
first appears, and the second considers the impact of Neoproterozoic tectonic changes on
climatic and biological oscillations.
The three independent lines of evidence—
ophiolites, blueschists, and UHP terranes—
agree that subduction tectonics began about
Neoproterozoic time, but did these related
manifestations begin synchronously? The oldest of each are not the same age—the oldest
ophiolites, ca. 1.03 Ga, are significantly older
than the oldest blueschists, ca. 800 Ma, and
these are older than the oldest-known UHP
terranes. Such a progression is expected from
our understanding of the subduction-driven
Wilson-supercontinent tectonic cycle and specifically from the sequence beginning with
Rodinia break-up and culminating in the amalgamation of a late Neoproterozoic supercontinent. Ophiolites appear first, as subduction
begins owing to lithospheric collapse. Next,
blueschists appear as collisions of oceanic plateaus, and arcs jam subduction zones and allow subducted blueschist facies oceanic crust
to be exhumed. UHP terranes form last, when
continents collide. Terminal collisions are accompanied by the deep subduction of continental crust (Liou et al., 2004), which rebound
to the surface when convergence ends. The sequence ophiolites → blueschists → UHP terranes is predicted when the global subduction
tectonic regime is established, and that is what
is observed in the Neoproterozoic rock record.
This sequence of events is also consistent with
the record of the seawater 87Sr/86Sr preserved
in carbonates, which rose dramatically ca. 600
Ma (Jacobsen and Kaufman, 1999). The 87Sr/
86Sr record provides independent confirmation
of Ediacaran continental collision by showing
that large tracts of continental crust were shedding radiogenic (high 87Sr/86Sr) detritus into
the oceans.
The second point to emphasize is that a revolution in the solid Earth system of this magnitude must have profoundly affected the exterior Earth system. Such wild climatic
fluctuations are documented for this time.
Neoproterozoic climate changed from intervals when perhaps the entire planet’s surface
was frozen, to sweltering greenhouses, and
back again. The carbon isotope record, faithful
monitor of the biomass, also fluctuated wildly
(Jacobsen and Kaufman, 1999). These oscillations are discussed as the ‘‘snowball Earth
hypothesis’’ (Hoffman et al., 2002). What initiated climatic instability in the Neoproterozoic is not known, but it may have been a
response to the initiation of subduction tectonics. Explosive volcanism would have increased dramatically as volcanic arcs first built
above subduction zones, injecting tephra, sulfur dioxide, and other gases into the strato-
sphere. Ash in the stratosphere may cool the
planet briefly, but sulfur gases would have a
longer effect via sulfuric acid aerosols (Robock, 2003). The primary chilling was volcanogenic. As an analog, Northern Hemisphere explosive volcanism increased
dramatically during late Pliocene time, and
this may have triggered Northern Hemisphere
glaciation (Prueher and Rea, 2001).
Chilling may have also resulted from decreasing CO2 in the atmosphere related to Rodinia breakup and indirectly to lithospheric
collapse. Continental fragmentation increased
runoff and hence consumption of carbon dioxide through continental weathering (Donnadieu et al., 2004). Rodinia breakup also witnessed the eruption of flood basalts, increasing
the weatherability of the land and consumption of atmospheric CO2 (Godderis et al.,
2003). Both processes would tend to boost
continental silicate weathering, consume atmospheric CO2, and help chill Earth.
CLOSING REMARKS
The hypothesis that the modern episode of
subduction tectonics began in Neoproterozoic
time is bound to be controversial. Nevertheless, evaluating this hypothesis and exploring
its implications for Neoproterozoic climate
and life will provide stimulating opportunities
for interdisciplinary research into the fascinating Neoproterozoic Earth system. Resolving this controversy also will provide new avenues for understanding fundamental
geodynamic questions, including what are the
driving forces of plate tectonics and how does
subduction begin?
ACKNOWLEDGMENTS
I thank G. Abers, W.G. Ernst, J.G. Liou, and S.
Maruyama for their thoughts on blueschists; A.
Kröner, H. Frimmel, B. Windley, V. Pease, J.-P. Liégeois, K. Hefferan, F. Alkim, and V. Khain for their
insights about Neoproterozoic ophiolites; A. Kröner,
J. Meert, and R. Van der Voo for their thoughts
about paleomagnetic evidence; and S. Self and N.
Miller for their thoughts about global cooling. I also
thank Yildirim Dilek, Mike Gurnis, and Norm Sleep
for thoughtful reviews and Kevin Burke for inspiration. This research is funded by National Science
Foundation grants OCE-0405651 and EAR0309799. This is University of Texas at Dallas Geosciences Contribution 1043.
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Manuscript received 16 November 2004
Revised manuscript received 31 January 2005
Manuscript accepted 4 February 2005
Printed in USA
GEOLOGY, July 2005